1. Field of the Invention
For supplying combustion chambers of self-igniting internal combustion engines with fuel, both pressure-controlled and stroke-controlled injection systems may be used. As the fuel injection systems, not only unit fuel injectors and pump-line-nozzle units but also common rail injection systems may be used. Advantageously, common rail injection systems (common rails) make it possible to adapt the injection pressure to the engine load and rpm. To attain high specific outputs and to reduce emissions from the engine, the highest possible injection pressure is generally required.
2. Description of the Prior Art
For reasons of strength, the feasible pressure level in common rail injection systems in use today is limited to about 1600 bar. To increase the pressure further in common rail injection systems, pressure boosters are.
European Patent Disclosure EP 0 562 046 B 1 discloses an actuation and valve assembly with damping for an electronically controlled injection unit. The actuation and valve assembly for a hydraulic unit has an electrically excitable electromagnet, with a fixed stator and a movable armature. The armature has a first and a second surface which define a first and second hollow chamber, and the first surface of the armature points toward the stator. A valve connected to the armature is capable of carrying a hydraulic actuation fluid from a sump to the injection device. Relative to one of the hollow chambers in the electromagnet assembly, a damping fluid can be collected there or drained off again from there. By means of a valve region that points into a central bore, the fluidic communication of the damping fluid can be selectively opened and closed in proportion to the viscosity of the fluid.
German Patent Disclosure DE 101 23 910.6 relates to a fuel injection device used in an internal combustion engine. The combustion chambers of the engine are supplied with fuel via fuel injectors which are acted upon via a high-pressure source; moreover, the fuel injection device of DE 101 23 910.6 has a pressure booster that has a movable pressure booster piston which divides a chamber, which can be connected to the high-pressure source, from a high-pressure chamber that communicates with the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a back chamber of the pressure booster with fuel and emptying this back chamber of fuel.
The fuel injector includes a movable closing piston for opening and closing the injection openings that point toward the combustion chamber. The closing piston protrudes into a closing pressure chamber, so that the pressure chamber can be acted upon by fuel. As a result, a force urging the closing piston in the closing direction is generated. The closing pressure chamber and a further chamber are formed by a common work chamber, and all the portions of the work chamber communicate permanently with one another for exchanging fuel.
With this embodiment, by triggering the pressure booster via its back chamber, it can be attained that the triggering losses in the high-pressure fuel system can be kept slight, in comparison with triggering via a work chamber that communicates intermittently with the high-pressure fuel source. Moreover, the high-pressure chamber is relieved only as far as the pressure level of the high-pressure reservoir, and not to the leakage pressure level. This improves the hydraulic efficiency, on the one hand, and on the other, a faster pressure buildup to the system pressure level can take place, so that the time intervals between injection phases can be shortened considerably.
In pressure-controlled common rail injection systems with a pressure booster, the problem arises that the stability of injection quantities to be injected into the combustion chamber, and especially the realization of very small preinjection quantities, which are required in a preinjection, is not reliably assured. This can be ascribed above all to the fact that the nozzle needle opens very fast in pressure-controlled injection systems. Therefore even very slight deviations in the triggering duration of the control valve can have a major effect on the injection quantity.
With a view to increasingly stringent demands in terms of emissions and noise in self-igniting internal combustion engines, further provisions in the injection system are required, in order to meet more-stringent limit values that are expected in the near future.